<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>

        #main{
            width: 500px;
            height: 500px;
            background-color: black;
            display: flex;
            justify-content: center;
            align-items: center;
        }

    </style>
</head>
<body>
<div id="main">
    <div id="a1"></div>
    <div id="a2"></div>
</div>
</body>
</html>
